The invention relates to a cationically stabilised silica dispersion, a method for its production and its use.
Aqueous silica dispersions are widely used in industry. For example, they are used in coating slips on paper, films and other printing media, as coatings on surfaces such as wood, plastic, metal, textiles and films for improving the mechanical and optical properties of the coating, in coatings on films for better separability or in abrasives and polishing agents for machining surfaces such as ceramics, glasses, glass ceramics and semiconductor wafers in the production of electronic components.
The isoelectric point (IEP) of SiO2 is a pH of 2. SiO2 particles in aqueous suspension are therefore negatively charged at a pH value of over 2, and are therefore anionically stabilised. To produce stable products containing silica it is necessary for the charge of the silica dispersion to have the same sign as the other components of the product, otherwise no stable end products are obtained. Anionically stabilised silica cannot therefore be used in products with cationically charged components.
In addition to the required stability of the composition, there are other reasons for using cationic dispersions, e.g. when the product to be produced is to contain cationic groups. For printing media, for example, it is necessary for the normally anionic dyes to be fixed on a cationic porous printed surface to achieve watertightness and high brilliance, among other things. There are also applications in which an agent is to be precipitated on substrates. In the case of negatively charged substrates cationic agents are naturally suited for the precipitation. For example, there are textile treatment agents in which cationic substances adhere to anionically charged fibres (fabric conditioners). Glass and ceramic surfaces, such as glazed ceramic surfaces, are also negatively charged. Compatibility with the binder is often a problem. Tests have shown that the addition of unmodified aerosol to PVA results in a sharp increase in viscosity.
On the other hand there are other metal oxide particles, e.g. of Ti or Al, that are cationically stabilised in an aqueous suspension, and that can therefore be used without problem in cationic products. However, the price of these metal oxides is much higher than that of SiO2. Since normal silica dispersions cannot be used in products with cationically charged components, as explained above, a modification of SiO2, which allows the use of SiO2 in products with cationically charged components, is of interest.
DE-T2-60106258 describes the use of particles produced hydrolytically in the gas phase, such as TiO2, Al2O, SiO2 for the production of an ink receiving coat, without a description of the treatment of the particles. The production of a cationically stabilised silica dispersion using SiO2 particles containing boron is described in DE-A-10311722.
DE-A-10033054 describes the stabilisation of a silica dispersion by means of cationic organic polymers. U.S. Pat. No. 6,777,039 describes the production of a coating for an inkjet printing medium by the addition of an aqueous solution of polyvinyl alcohol, an organic solvent and a tenside to a silica dispersion and a cationic polymer. U.S. Pat. No. 6,417,264 describes a silica dispersion which is dispersed with an organic cationic polymer in a polar solvent. U.S. Pat. No. 6,420,039 describes a silica dispersion in which SiO2 particles are brought into contact with an aluminium compound in order to achieve stabilisation.
EP-A2-1559750 describes the production of a cationically stabilised silica dispersion in a multi-step method in which an aqueous SiO2 dispersion is repolarised in a first step with an activating reagent such as aluminium chloride hydrate, and the reaction product modified with Al-containing material is treated in a further step with organosilanes. Direct addition of aminosilane to an aqueous solution of silica, without previously being treated with the activating reagent, is also described in this publication as a counter-example. However, according to the method described there, the silica dispersion gels immediately after the addition of aminosilane, so that the stabilisation of silica particles is not possible according to this method.